1. Field of the Invention
The present invention relates to a network device, a method for controlling the network device, and a network system, and more particularly to a network device, a method for controlling the network device, and a network system which multiplex TCP on a proxy device to speed up a communication.
2. Description of the Background Art
When a plurality of information communication devices communicate with each other, a transmission control protocol (TCP) defined by RFC 793 (“TRANSMISSION CONTROL PROTOCOL, DARPA INTERNET PROGRAM, PROTOCOL SPECIFICATION”, pages 3-6, sec. 1.5, “Flow Control”, http://datatracker.ietf.org/doc/rfc793) is generally used. The TCP is substantially a standard protocol on the internet, and installed in a large number of communication devices. With the use of the TCP, the information communication devices can communicate with each other regardless of the type of the information communication devices.
[WAN Speed-Up Device]
However, when a communication is conducted between a data center and a hub through a wide area network (WAN), an effective bandwidth is bottlenecked by a delay of the WAN from the restrictions of a flow control of the TCP, as a result of which even if a contracted bandwidth of the WAN is widened, the speed-up effect may not be obtained. Under the circumstance, there has been proposed a TCP speed-up technique for improving the flow control of the TCP in a network proxy device (device executing a communication by proxy). Also, the speed-up device of this type is called “WAN speed-up device”.
For example, JP-A-2003-69615 pertaining to a flow control improvement of the TCP discloses that “One or more routers 70a and 70b are arranged between a source host 50 and a destination host 60. The router 70a generates and transmits an acknowledge packet responsive to a packet transmitted from the source host 50, and terminates and divides a connection between the source host 50 and the destination host 60. An address of the destination host is used for a start address of the acknowledge packet. Also, a packet whose start address is an address of the source host is brought to the destination host 60. As a result, both of those hosts are made to recognize one virtual connection.” (refer to “technical solution”).
[Device for Transmitting TCP in a Lump]
Also, as another technical solution to the problem that the effective bandwidth is not obtained from the restrictions of the flow control of the TCP when a communication is conducted between the data center and the hub through the wide area network, there has been proposed a method for changing a communication control system of a transmitter side communication terminal, and multiplexing the TCP.
For example, JP-A-2004-260668 discloses that “There is provided a moving image transmission system including a transmitting device that transmits moving image data required to be reproduced in real time by TCP/IP, and a receiving device that receives the data transmitted from the transmitting device. The transmitting device transmits divided image data obtained by further dividing individual unit image data with one frame or a plurality of frames of the moving image data as one unit through a plurality of TCP connections, and the receiving device has a buffer part that proceeds with the processing for returning the divided image data received through the plurality of TCP connections to the original unit image data, and storage of the moving image data for the plurality of frames, and determines the division number of unit image data, and the number of TCP connections on the basis of a throughput per one TCP connection and an average bit rate of the moving image data.” (refer to “technical solution”).
Further, a method for applying the TCP multiplexing to proxy is also provided. For example, in US2005/025150 (Accelerating Network Performance by Striping and Parallelization of TCP Connections), “The Proxy application running on network accelerator 14-1 then assigns one or more of the persistent connections it has with the network accelerator 14-2 to handle the connection requested by Machine A.” is disclosed (refer to paragraph [0031] in “Detailed Description of the Invention”).